Silver halide photography usually involves the exposure of a silver halide photographic element with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. In order to sensitize the silver halide to other than the blue region, sensitizing dyes are used in the silver halide emulsion. Sensitizing dyes are chromophoric compounds (usually cyanine dye compounds). Their usual function is to adsorb to the silver halide and to absorb light (usually other than blue light) and transfer that energy via an electron to the silver halide grain thus rendering the silver halide sensitive to radiation of a wavelength other than the blue intrinsic sensitivity. However, sensitizing dyes can also be used to augment the sensitivity of silver halide in the blue region of the spectrum.
Most modern color photographic printing papers employ silver halide emulsions having a high chloride content in order to obtain rapid processing rates relative to silver bromide emulsions. The composition of these silver halide emulsions is usually AgClBr, where the percentage of bromide is very low, typically around 0.5% to 5%, and usually about 1%. The presence of small proportions of bromide enhances photoefficiency of the silver chloride emulsions and enhances the adsorption of sensitizing dyes to the emulsion surface while still allowing for rapid processing. However, even with small amounts of bromide present, some sensitizing dyes do not adsorb well to these emulsions resulting in poor spectral sensitization.
Color printing papers usually consist of at least three emulsions that are sensitized to blue, green and red light. Proper sensitization can be achieved by employing an appropriate sensitizing dye in each layer. Most common color photographic printing papers have a blue layer with a sensitizing dye that has a maximum sensitivity at about 480 nm.
Color photographic printing paper is intended to generate a print from a photographic color negative. An important quality characteristic of color paper or any color photographic element (including negatives) is color reproduction. Color reproduction is the ability to accurately portray the colors, or, more precisely, the hues of the original scene. It is known that using an emulsion in photographic elements which is blue sensitized with a dye to have a maximum sensitivity below 480 nm and low sensitivity at 480 nm, can lead to better color reproduction. This, and the importance of specific red and green sensitizations was described some time ago in U.S. Pat. No. 3,672,898. Subsequent patent publications such as U.S. Pat. No. 5,180,657 and US statutory invention registration H1243 describe color reproduction advantages with maximum blue sensitizations lying in various ranges which are below 480 nm.
However, the present inventors realized that using a shorter blue sensitizing dye can result in a speed loss. For color paper, one reason for this is that the energy output of the exposing device in many color paper printers decreases with decreasing wavelength in the blue region. Also, blue sensitive dyes have lower oscillator strengths than green or red sensitive dyes, resulting in relatively lower light absorption. Thus, providing blue sensitizing dyes with higher speeds, even for silver halide emulsions in color negative and color reversal elements, helps overcome this inherent deficiency.
Thus it would be desirable to provide a photographic element with a blue sensitized emulsion sensitized with a dye to have a maximum blue sensitivity at less than about 485 nm (particularly less than 480 nm) in order to obtain good color reproduction but which would still provide good speed.